Making alkali metal alloys for cathode lamps

ABSTRACT

A type of spectral source lamp has a hollow cup-shaped cathode, the interior of which is coated with the spectrally emitting element or elements. A technique for forming such a coating of an alloy of an alkali metal (or metals), with, say, tin in the presence of some boron is proposed, (resulting in higher melting points and lower vapor pressures, thereby allowing higher operating lamp currents and consequent spectral radiation intensity). The coating material may be conformed directly on the interior of the cathode cup (say, of titanium) by fusing an alkali metal borohydride with tin, thereby avoiding the need to handle pure alkali metal. The hydrogen gas liberated during alloy formation removes some of the contaminants (e.g., oxides). A boron-containing, glassy slag may be readily separated from the alkali metal alloys. Specific examples in which the alkali metal component is sodium, potassium, or a mixture of sodium and potassium are disclosed. The other metal may be, for example, tin or lead.

United States Patent inventors John W. Vollmer Original application July27, 1967, Ser. No. 656,564, now Patent No. 3,560,790, dated Feb. 2,1971. Divided and this application Apr. 27, 1970, Ser. No. 32,220

MAKING ALKALI METAL ALLOYS FOR CATHODE LAMPS 9 Claims, 1 Drawing Fig.

US. Cl 29/25.]7, 75/134 A, 75/135, 75/167, 75/175 R Int. Cl l-l0lj 9/02,C22c 1/00 Field of Search ..75/l35, 134

A, 134 P, 167,175 R; 29/25.]7

[56] References Cited UNITED STATES PATENTS 2,717,206 9/1955 Shapiro75/167 Primary Examiner-L. Dewayne Rutledge Assistant Examiner-E. L.Weise Attorney-Edward R. Hyde, Jr.

ABSTRACT: A type of spectral source lamp has a hollow cupshaped cathode,the interior of which is coated with the spectrally emitting element orelements. A technique for forming such a coating of an alloy of analkali metal (or metals). with, say, tin in the presence of some boronis proposed. (resulting in higher melting points and lower vaporpressures. thereby allowing higher operating lamp currents andconsequent spectral radiation intensity). The coating material may beconformed directly on the interior of the cathode cup (say, of titanium)by fusing an alkali metal borohydride with tin, thereby avoiding theneed to handle pure alkali metal. The hydrogen gas liberated duringalloy formation removes some of the contaminants (e.g., oxides). Aboron-containing, glassy slag may be readily separated from the alkalimetal alloys. Specific examples in which the alkali metal component issodium, potassium, or a mixture of sodium and potassium are disclosed.The other metal may be, for example, tin or lead.

MAKING ALKALI METAL ALLOYS FOR CATHODE LAMPS This application is adivision of our copending U.S. Pat. application Ser. No. 656,564, filedJuly 27, 1967, now US. Pat. No. 3,560,790 dated Feb. 2, 1971.

This invention relates to a method of improving hollow cathode lamps ofthe type used as sources of spectral radiation. More particularly theinvention concerns the preparation of hollow cathodes for such lamps, inwhich the active material on the interior of the hollow cathode holderincludes at least one alkali metal.

INTRODUCTION One type of source of spectral radiation (which is usefulin spectroscopic analysis, for example by means of an atomic absorptionspectrometer) is the hollow cathode lamp. In such lamps the cathode iscup-shaped and includes on (at least a substantial portion of) itsinterior surface a material including the element or elements, havingthe spectral radiation characteristic desired. For those elements havingsuitable physical characteristics (such as melting point, vapor pressureand electrical characteristics), the spectral element may be, forexample, a coating on the interior of a hollow cathode holder of anothermetal. If the element for which the spectral radiation is desired has,for example, extremely low-melting point or high-vapor pressure at theoperating temperature of the lamp, other techniques must be utilized.One such technique is the information of an alloy of the desired element(or elements) with other metals.

The alkali metals as a class have very low-melting points and veryhigh-vapor pressures relative to the normal operating temperature of thecathode of the lamp (around 400 C.). [t has already been proposed to usebinary alloys of the alkali metals (for example, sodium and potassium)with for example, lead, and utilize the resulting alloy (e.g., NaPb andKPb,) as the interior surface of a hollow cathode. Such prior techniquedoes not provide a complete solution to the problem in that theresulting binary (sodium-lead and potassium-lead) alloys still haverelatively low (approximately 325 C, for NaPb melting points, therebynecessitating relatively low-operating temperatures (and therefore bothlow current and radiation intensity). Additionally the formation of suchalloys in situations utilized in hollow cathode lamp production ispractically difficult because of the well-known problems in handling theextremely chemically active alkali metals.

The present invention greatly facilitates manufacture, in that thematerials initially alloyed are both safer and require less expensiveapparatus in their handling, both prior to and during the alloyingprocess. Broadly the invention utilizes one or more alkali metalborohydrides and a suitable additional metal (for example, tin) to forman alloy containing the desired alkali metal, some boron, and theadditional metal. The resulting alloy also has a somewhat higher meltingpoint and lower vapor pressure at, say, 350 C. than the correspondingbinary alloys.

Accordingly, an object of the invention is the provision of a simpler,more economical method of manufacture of a hollow cathode assembly foran alkali metal spectral radiation lamp.

Other objects, advantages and features of the invention will becomeobvious to one skilled in the art upon reading the following detaileddescription in conjunction with the accompanying drawing, in which:

The sole FIGURE is a cross section through a hollow cathode assembly ofthe invention, including an interior coating of the alkali metal, boron,and additional metal alloy.

DESCRIPTION The drawing illustrates a finished hollow cathode assemblyin which a hollow cathode cup holder 22 (of, for example, pure titanium)has a coating 30 of an alkali metal, boron, and additional metal alloysubstantially covering the interior surface 24 of both' the cylindricalsidewall portion 26 and the heavier bottom portion 27 of the holder.Such conventional hollow cathode holders are provided with a reducedportion 28 having a recess 29 for engagement with a pin (not shown) ofthe lamp in which they are used, which pin provides both mechanicalsupport for and the (negative) voltage connection to the cathodeassembly. The tertiary alkali metal alloy at 30 may either by prealloyedand then cast within the hollow cathode cup 22, or both the alloying andcasting may be done in the same holder 22 intended to be utilized in thefinished assembly, as will appear hereinafter.

A general description of how the alkali metal alloy coatings accordingto the invention may be made is given, followed by three specificexamples, utilizing different alkali metals. ln the immediatelyfollowing description the alkali metal will be assumed to be potassiummerely for simplicity of expression; as will be seen not only maydifferent alkali metals be used, but even mixtures of different alkalimetals.

A small quantity of the alkali metal borohydride (e.g., KBl-l.) ispositioned at the bottom of a hollow cathode cup (having its reduced end28 lowermost) or a suitable crucible of similar shape (which cruciblemay be made for example of graphite). The alkali metal borohydrides (inparticular, potassium and sodium borohydrides) are substantiallycompletely stable in dry air at room temperatures (these particularborohydrides have been maintained in (dry) air-filled vials for severalweeks without any noticeable deterioration through chemical reaction).For long periods of storage, or at elevated temperatures (as in thesucceeding manufacturing steps) the borohydrides should preferably bemaintained under an inert atmosphere (for example, argon or at least dryair). For short term periods (such as during weighing or other simplemanipulative steps), the borohydrides may be exposed to normal ambientair (especially of only moderate humidity) without any appreciabledecomposition occurring. A substantially larger (on the order of tentimes as much by weight) quantity of relatively pure tin is then placedon top of the alkali metal borohydride, and the entire assembly heatedto cause first melting of at least the tin and then decomposition of thealkali metal borohydride in a controlled manner, thereby evolvinghydrogen. The inert atmosphere is preferably constantly changed so as toflush away the evolved hydrogen.

A convenient apparatus is an enclosed centrifuge having an externalinduction heater and having inlet and outlet connections for theconstantly flushing, say, argon gas. Constant moderate current issupplied to the induction heater until the tin melts (at 232 C.)completely. The current is then slowly raised until evolution of thehydrogen (indicating the decomposition of the borohydride) starts. Therate of decomposition should be controlled by (manual) adjustment ofheater current, or more simply by turning the heater current switch onand off, to avoid violent hydrogen release and the consequent loss ofmaterial over the upper edge of the container (e.g., 23 of theillustrated hollow cathode holder). During the entire heating operationthe container and its contents are preferably slowly spun by thecentrifuge to assist in mixing of the ingredients and escape of thehydrogen gas.

The passage of the hydrogen gas through the molten tin has the desirableefiect of reducing any tin oxide which may be present (because ofsurface oxidation of the tin in its original form). After the bubblingstops (indicating that all of the alkali metal borohydride has given upits hydrogen), the heating current is completely turned off, and thegraphite crucible, hollow cathode holder, or other container is cooled.A glassy coating or slag is formed substantially on the upper surface(e.g., at 32) of the alloy, such slag 34 including a substantialproportion of boron compounds. This slag may be readily removed from themetallic alloy (e.g., of potassium, boron, and tin).

Although it is impossible to recast the alloy in the same hollow cathodeholder (when such is used as the container in the foregoing steps),preferably the alloy is recast into a clean new cathode cup (of titaniumfor the exemplary alloy) either at this stage or at a later stage oflamp assembly. The desired shape of the final alloy coating at 30 may beobtained either by centrifuging or by nutating (i.e., turning about awobbly generally vertical axis) the cathode or the lamp into which ithas already been installed while the alloy is molten, and then coolingto solidify the coating.

SPECIFIC EXAMPLES Example 1: Sodium specific approximate useful examplerange sodium borohydride 0.123 007041165 tin 0.877 0330-0835 Aspreviously stated, the container with the sodium borohydride in thebottom and the tin thereover is placed within a flushing inert (e.g.,argon) atmosphere. Thereafter heat is supplied to first melt the tin andthen to decompose the sodium borohydride (at about 500 C.), moderatingthe rate of hydrogen evolution to avoid loss of material; thetemperature is finally raised slightly to insure a complete melting ofthe alloy. All of the heating steps are preferably done with the hollowcathode cup or other container being slowly spun by the centrifuge. Uponcooling, the previously noted glassy layer 34 tends to form as adiscontinuous dispersion or group of particles over the sodium, boron,tin alloy surface (i.e., 32). These particles may be readily removed bymechanical means (i.e., physical scraping), and the tertiary sodium,boron, tin alloy is then preferably recast in a titanium hollow cathodeholder (22) (which may be identical to container as used in the previoussteps).

For a total of one gram of initial ingredients (and therefore 0.l23grams of sodium borohydride), approximately 0.0l4 grams of hydrogen willbe released. This must hydrogen would occupy approximately 150 cc. atstandard conditions (atmospheric pressure and C. or 273 K. and willoccupy somewhat less than 500 cc. at the elevated temperature(approximately 500 C. or 773 K.) utilized during the bubbling period. Arelatively fast flushing of (say argon) inert gas is thereforepreferably used to insure substantially complete removal of the hydrogengas as a relatively low, safe concentration of hydrogen in the outflowgas.

Example ll: Potassium A similar potassium, boron and tin tertiary alloymay be formed by utilizing an analogous technique, using a somewhatsmaller amount (by weight) of potassium borohydride The followingproportion of original ingredients (again normalized to a total of onegram) may be used:

specific approximate example useful range potassium borohydride 0.0800050-0140 tin 0.) 20 0950-0360 (i.e., 32) of the final alloy, and maystill be removed by simple mechanical means; and less hydrogen isevolved per gram of total mixture for the same weight percentage ofpotassium borohydride, namely, about 65 cc. at standard conditions(about cc. 500 C., (773 K.)), or about 210 cc. at 600 C. (873 K.).Except for the somewhat different scraping procedure of the slag and thesomewhat lower inert gas flushing rate useable, the potassiumborohydride and, say, tin are processed in the same way to form thetertiary (potassium, boron and tin) alloy and the final coating.

Example lll: Sodium-Potassium Mixed Alloy specific approximate exampleuseful range sodium borohydride 0.062 0035-0120 potassium borohydride0.040 0025-0100 tin 0.898 0940-0780 The alloying technique is againessentially identical to that of the general description in example Iwith the following minor differences. The total amount of evolvedhydrogen (and therefore the minimum sufficient inert gas flushing rate)will be intermediate between those of examples I and ll. Similarlyalthough the slag formation is somewhat different in form, removal ofthese boron compounds is essentially no more difficult than in examplesI and I].

As previously mentioned, during the initial alloying process (for all ofthe above examples and all analogous alloy formations), the temperatureshould be carefully raised slightly after the apparent completion of thedecomposition (and alloying) of the borohydride (i.e., after bubbling ofhydrogen ceases), to insure actual complete consumption of the alkalimetal borohydride(s). On the other hand, the temperature should never beraised much above that necessary to cause the particular result desired(e.g., melting of the additional metal, say, tin; decomposition of theborohydride at moderate rate; and melting of the final alloy duringfinal casting); Such moderation in temperature lessens the possiblelosses of the alkali metal (as vapor) during the various manufacturingstages.

Alternative and Conclusion Although all of the specific examples givenabove utilize tin as the additional metal, other metals may be usedinstead. The additional metal should have reasonably satisfactoryphysical characteristics, the ability to alloy with the alkali metal inthe presence of boron, and be free of any spectroscopic interferencewith the alkali metal(s) spectral line emission. An ex ample of a metalhaving such properties, which may therefore be substituted for the tinin the above examples, is lead (which has in fact been successfullytried).

In both examples I and ll the alkali metals (sodium and potassiumrespectively) are initially introduced so as to form approximately 4-] 1percent (say 6 percent) by weight of the initial ingredients. In eachcase however a measurable but relatively small amount of the alkalimetal is lost in the form of the boron compounds in the slag and perhapseven as lost metal during the alloying process. In general the amount ofthe alkali metal (i.e., the sodium of example I, the potassium ofexample ll and the total of sodium, and potassium in example Ill) willbe reduced from, say, about 6 percent to approximately 2 to 4 percent byweight in the final alloy.

Similarly a substantial proportion of the boron is lost (primarily inthe slag material) so that its original proportion is probably halvedduring the manufacturing process. The residual boron, although typicallypresent as only a fractional percentage (by weight) in the total finalalloy, nevertheless has an appreciable effect in raising the meltingtemperature of the alloy (relative to a similar but boron-free alloy)and moderating the vapor pressure (at, for example, 350 C.) of thealkali metal.

The invention therefore provides a relatively simple technique forproviding an alkali metal alloy for a hollow cathode having desirablycharacteristics (and additionally provides a somewhat improved alloy forthis purpose having a somewhat additional melting point and lower vaporpressure than the most closely related previously used alloys, i.e., thebinary alloys of the alkali metals without any boron content. Theinvention process entirely avoids the handling of the pure alkalimetal(s) and the attendant problems and hazards, as well as requiringlittle equipment. It therefore is particularly suitable for formingrelatively small quantities of the alloy, for use, for example, inmaking only one or a few spectral radiation lamps at at time. Thehydrogen (and perhaps the boron as well) released during alloy formationreduced the oxides of the additional metal (often present on itssurface); in any event elimination of existing oxide from the finalalloy has actually been observed when either tin or lead (having somesurface oxidation) has been the additional" metal used.

Although three specific examples involving two alkali metals and amixture thereof have been specifically described, it will be obvious tothose skilled in the art that other alkali metals and mixtures thereofmay be used to form the analogous alloys. Similarly other additionalmetals (having the requisite moderately low-melting points and otherdesirable alloying properties, as well as exhibiting no spectroscopicinterference with the alkali metal emission) may be utilized besides tinand lead. Further although specific proportions have been given in theexamples, obviously the relative proportions may be varied over arelatively large range. Because of these and other obviously possiblevariations, the invention is not limited to any of the details of anyone or more of the exemplary embodiments specifically disclosed; on thecontrary the invention is defined solely by the scope of the appendedclaims.

We claim:

1. The method of forming an alkali metal alloy adapted for use as aspecially emitting cathode material of a radiation source lamp of thetype useful for spectroscopic analysis, comprising:

placing a minor proportion of at least one alkali metal borohydridewithin a container;

placing a major proportion of an additional metal within the samecontainer, said additional metal having a melting point which is lowerthan the decomposition temperature of said borohydride;

maintaining a substantially inert atmosphere in direct contact with saidborohydride during the following steps:

heating the original ingredients, comprising said borohydride andaddition metal, in a controlled manner so as to cause melting of saidadditional metal, and then gradual decomposition of said borohydride,thereby evolving hydrogen at a controlled rate;

continuing said controlled heating at a temperature somewhat above thedecomposition temperature of said borohydride until substantiallycomplete decomposition has occurred, and consequent hydrogen evolutionhas ceased,

thereby forming an alloy containing a minor proportion of alkali metalsof the original borohydride and a major proportion of said additionalmetal.

2, The alloying method of claim 1, in which:

said original alkali metal borohydride comprises sodium borohydride,

whereby a sodium alloy is ultimately formed.

3. The alloyin method of claim 1 in which: said original a kali metalborohydride comprises potassium borohydride,

whereby a potassium alloy is ultimately formed.

4. The alloying method of claim 1, in which:

said original alkali metal borohydride comprises a substantial quantityof each of sodium borohydride and potassium borohydride,

whereby a mixed alloy of both sodium and potassium is ultimately formed,which emits spectral line radiation characteristic of both sodium andpotassium when used as a cathode coating material in a source lamp.

5. The alloying method of claim 1, in which:

said additional metal comprises lead.

6. The alloying method of claim 1, in which:

said additional metal comprises tin.

7. The alloying method of claim 1, in which:

the surface of the ultimately obtained alkali metal alloy is cleaned ofthe slaglike undesirable byproducts also formed during the alloyingprocess,

and the resulting substantially pure alkali metal alloy is then recastinto the interior of a hollow cathode holder to form a spectrallyemitting coating thereon.

8. The alloying method of claim 1, in which:

said container comprises a hollow cathode holder,

said holder is rotated at a substantial rate at least during the end ofthe continuation of said controlled heating stage,

and said rotation is maintained after said heating steps are complete,until the ultimately formed tertiary alkali metal alloy has at leastpartially cooled,

whereby said ultimately formed alloy, while still molten, covers asubstantial part of the interior surface of said hollow cathode holder,and then cools thereon as a solid coating.

9. The alloying method of claim I, in which:

the total amount of said alkali metal borohydride comprises more thanabout 4 percent and less than about 20 percent, and said additionalmetal comprises at least about percent by weight of the total originalingredients.

2. The alloying method of claim 1, in which: said original alkali metalborohydride comprises sodium borohydride, whereby a sodium alloy isultimately formed.
 3. The alloying method of claim 1, in which: saidoriginal alkali metal borohydride comprises potassium borohydride,whereby a potassium alloy is ultimately formed.
 4. The alloying methodof claim 1, in which: said original alkali metal borohydride comprises asubstantial quantity of each of sodium borohydride and potassiumborohydride, whereby a mixed alloy of both sodium and potassium isultimately formed, which emits spectral line radiation characteristic ofboth sodium and potassium when used as a cathode coating material in asource lamp.
 5. The alloying method of claim 1, in which: saidadditional metal comprises lead.
 6. The alloying method of claim 1, inwhich: said additional metal comprises tin.
 7. The alloying method ofclaim 1, in which: the surface of the ultimately obtained alkali metalalloy is cleaned of the slaglike undesirable byproducts also formedduring the alloying process, and the resulting substantially pure alkalimetal alloy is then recast into the interior of a hollow cathode holderto form a spectrally emitting coating thereon.
 8. The alloying method ofclaim 1, in which: said container comprises a hollow cathode holder,said holder is rotated at a substantial rate at least during the end ofthe continuation of said controlled heating stage, and said rotation ismaintained after said heating steps are complete, until the ultimatelyformed tertiary alkali metal alloy has at least partially cooled,whereby said ultimately formed alloy, while still molten, covers asubstantial part of the interior surface of said hollow cathode holder,and then cools thereon as a solid coating.
 9. The alloying method ofclaim 1, in which: the total amount of said alkali metal borohydridecomprises more than about 4 percent and less than about 20 percent, andsaid additional metal comprises at least about 80 percent by weight ofthe total original ingredients.